lens to film plane precision for wide angle lens

quixotic · Dec 8, 2016

I have two Mamiya Press film backs: one for 6x9 and one for 6x7. The distance from the film plate to the camera mating surface for the former is 3mm exactly (when set to 120 film). The distance for the 6x7 is 3.25mm (also at 120). The 6x9 distance is perfect because that's what came with the camera (a Plaubel Proshift) and the photos come out tack sharp.

I can get the 6x7 distance down to 3.1mm by changing the film plate to 220. So I guess the question is: for a 47mm lens, will the difference of 0.1mm in the lens to film plane distance make any difference in sharpness? Or should I try to install some shims?

Thanks in advance.

paul ron · Dec 8, 2016

i dont work on press cameras but..... try this with the lens removed, back open n mounted on the camera....

measure the distance from the inside of the film mask to the edge of the lens mount flange?

still see that difference?

the preasure plate spring have adjustment screws or perhaps bent out of shape?
does the preasure plate toutch the film mask?
check the rollers?
does the film sit flat on the film mask?

i doubt the difference will make any difference in focusing, but i like things to be consistant.

i found the light seal flanges change in some camera interchangeable backs n may be the cause of your problem.

btw the odd back... seals rotted or missing?... doent need seals?

im not familiar with your backs so im sorry if it seems im guessing... i am.

Kirks518 · Dec 9, 2016

Remember, 120 has a paper backing which will account for some of that distance. But as paul ron says, it's the distance from the flange to the film rails, and not to the pressure plate. As long as the pressure plate is holding the film against the film rails, you won't have problems.

It sounds like you haven't run a roll through at 6x7. I'd run a roll first before monkeying with anything.

rthollenbeck · Dec 9, 2016

The 47xl has tremendous DOF. Unless you are shooting wide open(probably not even then). I doubt that makes any difference in your focus. A lot of wide angle cameras are focus by guess. You may say well that's "out front" but it still translates to laditude at the film plane as well. If you were using a long lens that maybe of more concern. If you have a piece of ground glass you can place it on the film guides and check it out for yourself. If your shooting something and your concerned stop it down.

moto-uno · Dec 9, 2016

I have made a number of ground glass pieces that fit on the film mask of many of my various cameras.
This has allowed me to check that the rangefinders in these cameras are accurate . With an 8x loupe I
can easily verify it and then know that it's my sloppy focusing and not the camera .( They're very easy to make .) And of course with a removable back you place the glass in the removable back.
Peter

ic-racer · Dec 9, 2016

You can use 1/p + 1/q = 1/f to figure out how far the subject distance will be off (the thin lens equation).
If p = 0.1mm then q = how far of subject distance will be off.

You can also use this equation to approximate the aperture needed to cover that focal spread.
N = D/(2c)

N = f stop number
D = 0.1mm
c = circle of confusion size you can tolerate

Maris · Dec 9, 2016

Wideangle lenses like the 47mm have lots of depth of field but they have very stringent depth of focus tolerances. If the film plane is 0.25mm back from where it should be a 47mm lens apparently focussed on infinity will actually be focussed on 11 metres. This might not be acceptable if the picture is to be greatly enlarged.

ic-racer · Dec 9, 2016

In practical terms, what do I do? Since my Horseman 6x9cm and Horseman 4x5in cameras are almost the same size and take the same lenses, I frequently go to the 4x5 camera because I can never be comfortable the rollfilm is holding perfectly flat. Realize that in spite of all your work to get the pressure plate in the right spot, the film can spontaneously bulge forward. Especially if there has been considerable time between exposures.

shutterfinger · Dec 9, 2016

Large Format film holder specifications state the edge of the holder to film surface to be .197 ± .007 inch for 2x3 through 4x5. In testing I have found these measurements to apply to roll film holders as well.
.197 inch = 5.0038 mm; .007 inch = 0.1778 mm
Sheet film is .007 inch thick.
Kodak and Ilford 120 roll film is .0045 inch (4.5mil) thick.
Fuji 120 color film is .0035 inch (3.5mil) thick
120 backing paper is .006 inch thick.

I believe your 6x7 roll holder is in tolerance.

John Koehrer · Dec 9, 2016

GG on the film guides won't help with setting of the pressure plate.
The rails don't move.

rthollenbeck · Dec 9, 2016

Maris said:
Wideangle lenses like the 47mm have lots of depth of field but they have very stringent depth of focus tolerances. If the film plane is 0.25mm back from where it should be a 47mm lens apparently focussed on infinity will actually be focussed on 11 metres. This might not be acceptable if the picture is to be greatly enlarged.

While truly would assume you completely correct from a scientific position......My educated guess from a practical standpoint leads me to believe in practice your information leads one to a false concussion about out of focus.
So follow me.... the super wide or the 47xl in a focus mount: They are both commonly focused by guessing a distance. The lens moves in an out on the helical mount. It's probably safe to say in general- even the distance guess is wrong. So the distance from the film plane varies and is quite often wrong. There is so much DOF most shots still work out.
I have to believe after looking at the distance scale and the corresponding movement of the lens on my own 47xl that it would be comon to be out by so much as 1mm when guess focusing.......and yet I don't rember ever getting a less than satisfactory shot.

frank · Dec 9, 2016

I understand that wide angle lenses have greater DOF, but do they also have greater depth of focus compared to the normal lens? I think it's less.

Not certain of the optical physics, but wide angle lenses have light rays striking the film at a lesser angle than the normal and longer lenses.

quixotic · Dec 9, 2016

Thanks everyone for the replies. I did some more measurements. The distance from the flange surface to the film rail differs in the two film backs by about 0.1mm's. However, on the film back that I know really works well (by the sharpness of the photos), the distance from the pressure plate to the film rail (0.3mm) is 0.1mm greater than the thickness of the film plus backing paper (0.2mm). So the film in the demonstrably adequate back is able to flop around by about 0.1mm anyway. So I guess I shouldn't worry too much about that variation in the lens to film plane distance (also 0.1mm).

I initially suspected that rthollenbeck was correct about the wide latitude of the 47mm lens, but now I feel even more relaxed about the whole issue.

quixotic · Dec 10, 2016

Well, I ran a roll of film through it. Wide open (5.6), it's a bit fuzzy, at f8, less so, but by f16, it's just as tack sharp as the other film holder (for some reason, I forgot to test at f11). It's hard to compare the 5.6 shot with anything taken with the 6x9 holder, since for that camera, it's not an aperture that I use much anyway (99% of my shots with that camera are done in the middle range of apertures, and on a tripod. In any event, I'm just happy that I can now use the Proshift and 1) get wide shots instead of superwide, 2) get a couple of more shots per roll, and 3) put the results in Gepe slide holders (the largest they make is 6x7).

ic-racer · Dec 10, 2016

The center field of the lens should be very, very sharp when the lens is wide open. Low in contrast and MTF, but very sharp to the eye. Your issue is probably of greater magnitude than you initially measured.

paul ron · Dec 11, 2016

quixotic said:
Well, I ran a roll of film through it. Wide open (5.6), it's a bit fuzzy, at f8, less so, but by f16, it's just as tack sharp as the other film holder (for some reason, I forgot to test at f11). It's hard to compare the 5.6 shot with anything taken with the 6x9 holder, since for that camera, it's not an aperture that I use much anyway (99% of my shots with that camera are done in the middle range of apertures, and on a tripod. In any event, I'm just happy that I can now use the Proshift and 1) get wide shots instead of superwide, 2) get a couple of more shots per roll, and 3) put the results in Gepe slide holders (the largest they make is 6x7).

what you focused on is fuzzy?

are you comparing 2 different cameras or 2 different backs on the same camera?

you viewfinder may not be accurate on one of those cameras. its more common for a viewfinder to be off rather than a film back.

gg on the film rails, focus a well lit target and check the gg film plane?

quixotic · Dec 11, 2016

Sorry, I should have mentioned that the camera has no ground glass or rangefinder. Focusing is via the distance numbers on the lens only (zone focusing?). So, even though I used a tape measure to get the distance from object to the film plane, I may not have precisely turned the distance scale ring. Plus, I've always heard that the sweet spot on lenses were usually in the middle of the available apertures (not sure if that's the case with the 47mm).

I guess I should also define fuzzy. When I view the negative under a 20x stereo microscope, the image at 5.6 is obviously not as sharp as it is at f16. But on an 8x10 print, I might not notice any difference.

shutterfinger · Dec 11, 2016

One sure way to check the two roll film holders is to set the camera up on a tripod, set focus center frame on a target such as a fence or brick wall with the camera at a 22.5° angle to the subject, then make an exposure with both roll film holders without changing anything on the camera or moving its position with the lens wide open. The point of sharp focus should be the same for both but if one is off as you suspect then the direction and amount of focus shift will be obvious.

lens to film plane precision for wide angle lens

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